The invention relates generally to the field of seagoing tank vessels, and in particular, to a rebuilt double hull tanker and a method of rebuilding an existing single hull tanker into a rebuilt double hull tanker.
The shipping and cargo moving industry is continually faced with customer demand for new and improved tank vessel designs and for new and improved methods of modifying the design of existing tank vessels. Substantial cost savings can be realized by a vessel owner in modifying or rebuilding existing tank vessels to incorporate improvements in tank vessel designs or otherwise extend the life of the tank vessel rather than paying the cost of buliding a new tank vessel.
In addition, new governmental and environmental regulations place certain restrictions and requirements on tank vessel owners and operators. These new or required designs must be capable of securely holding a cargo and also of being seaworthy. At the same time, a tank vessel must comply with shipping and environmental requirements and regulations.
Conventional tankers comprise a tank vessel having a single hull design. This type of hull construction provides a single outer hull or skin that provides structural integrity and acts as a boundary between the operating environment of the tanker (e.g., the sea) and the cargo and internal structure of the tanker. The single hull typically includes a shell having a bottom, a port side, a starboard side, a bow, a stem, and a plurality of bulkheads and internal stiffening frames that support and strengthen the shell of the hull.
Tankers are vessels specially designed to carry liquid or fluid-type cargoes, such as petroleum or chemical products. A problem unique to single hull tankers is that damage to the tanker""s hull may lead to rupture of the tanker""s cargo tanks and thus spill or leakage of the cargo. This results not only in the loss of cargo, but also in pollution of the marine environment and accompanying coastline.
As a result of the recent heightened environmental awareness and several shipping mishaps, new governmental regulations have been implemented requiring the use of double hulls on designated tank vessels in U.S. waters out to the 200 mile economic zone limit. These double hull requirements are contained in the Oil Pollution Act of 1990 (OPA-90) and have been incorporated in U.S. Coast Guard regulations. In part, OPA-90 requires that all new tank vessels constructed under contracts awarded after 1990 must have double hulls and that all existing single hull tank vessels engaged in the marine transport of oil and petroleum products be rebuilt with double hulls or be retired between the years 1995 and 2015, depending on the size and age of the tanker. The U.S. rules closely parallel those of the International Maritime Organization, which rules apply worldwide.
This has created a great burden on carriers having existing single hull tankers. These single hull tankers will either have to be rebuilt to incorporate a double hull design at great cost to the carrier, or the tankers will have to be retired, in many cases years before the end of their economically useful life.
Double hull designs have been used in the construction of newer tankers in an effort to comply with the requirements of the OPA-90. These double hull vessels typically have an outer hull and an inner hull. The outer hull and the inner hull each have shell plating that forms the structural integrity of the hull. A combination of transverse and longitudinal framing is provided between the inner and the outer hull to help strengthen the shell plating.
The idea behind a double hull is that the structural integrity of the outer hull may be breached without breaching the inner hull. Therefore, the outer hull may be breached, i.e., opened to the sea, while the cargo would remain securely contained within the inner hull. Thereby, a potential cargo spill will have been avoided. Typical cargos that have spilled in the past to cause environmental mishaps include cargos such as oil, petroleum, chemical, or other hazardous materials. Of course the provision of a double hull adds to the complexity and cost of new construction.
U.S. Pat. No. 5,218,919, entitled xe2x80x9cMETHOD AND DEVICE FOR THE INSTALLATION OF DOUBLE HULL PROTECTION,xe2x80x9d issued on Jun. 15, 1993 to Krulikowski et al. describes the construction of an auxiliary hull, exterior to the primary hull of a ship, which has the capacity to absorb impact energy preventing primary hull puncture, which may be retrofitted to existing single hull ships. However, this external fitting of a new auxiliary hull outside the entirety of the existing single hull to form a double hull is costly and significantly changes the operational characteristics of the vessel. Installing a new auxiliary hull over the existing bottom hull also affects the draft and lowers the baseline of the tanker, significantly affecting flow into the propeller. Also, this design does not meet OPA-90 requirements for minimum hull spacing.
U.S. Pat. No. 5,189,975, entitled xe2x80x9cMethod for Reconfiguration Tankers,xe2x80x9d issued Mar. 2, 1993 to Zednik et al. describes a method for converting a single hull tanker to a mid-deck configuration. As disclosed by Zednik et al., the mid-ship cargo section of the tanker is cut longitudinally along a horizontal plane well below the normal laden waterline. A spacer member including a new transverse mid-deck is interposed between the lower and upper portions of the mid-ship cargo section. A tank vessel having a mid-deck configurations are comprised of vertical cargo tanks (one above the other) and double sides, but do not include double bottoms and therefore are not as effective as double hulls, and do not meet OPA-90 requirements (e.g., this type of construction in the U.S. does not constitute a double hull and is considered to be a single hull).
Japanese patent JP 361024685 A, entitled xe2x80x9cMethod of Reconstructing Existing Tanker into Double Hull Tanker,xe2x80x9d and Japanese patent 61-24686 both show a method of reconstructing an existing tanker into a double hull tanker wherein a new inner hull and new inner side hulls are installed inside the existing outer plating. However, this method decreases the cargo carrying capability while at the same time also increases the draft of the vessel due to the increased weight of the double hull, both of which are undesirable.
U.S. Pat. Nos. 6,170,420 B1 and 6,357,373 B1 disclose internal rebuilt double hull vessels and methods of accomplishing same. These patents disclose a process wherein the topside decking is cut and removed and a new inner hull is disposed internally over the existing single hull to form the new double hull. While this internal double hull process works well for barges, it is not as effective for tankers for several reasons including (1) the use of a raised trunk to help maintain the same cargo carrying capacity on a rebuilt barge causes more visibility and operational issues on tanker than on a barge; (2) tankers are generally three tanks across instead of two, which causes structural complications with the new double sides not normally experienced with barges; (3) tankers typically have more services (fuel, oil, electricity, water, cargo handling, ship handling, etc.) that would be disrupted during a rebuild by cutting up the deck to create a raised deck than would a typical barge; (4) the increase in draft due to the additional weight of the new double hull would be greater for a typical tanker than a typical barge due to hull shape of a tanker, which would adversely affects marketing and may limit the cargo in several ports; (5) the extra steel weight on a tanker would represent lost cargo weight unlike the barge where the extra draft is allowed by regulation and compensates for the extra steel weight; (6) hull bending moment issues arising from the concentrated weights in the tanker""s engine room which typically do not exist on a barge; and (7) the method used on a typical barge retrofit is difficult to accomplish on a typical tanker due to access and interference problems and modification of existing ship structure and piping.
Therefore, a need exists for a rebuilt tanker having a double hull having substantially the same cargo carrying capability at substantially the same or a reduced draft. The need also exists for an improved method of rebuilding an existing single hull tanker into a rebuilt double hull tanker that minimizes disruptions in existing ship services and accounts for access and interferences problems and modifications of existing ship structure and piping.
The present invention is directed to a double hull tanker rebuilt from an existing single hull tanker. The rebuilt double hull tanker includes a new double bottom hull and a new double side hull (e.g., port and starboard) formed over at least a cargo carrying portion of the rebuilt tanker. The new double bottom hull includes an inner bottom hull formed from new inner bottom plating disposed internally and in a spaced apart relationship with an outer bottom hull formed from the existing bottom plating. The new port and starboard double side hulls include a new outer side hull formed from new outer side plating disposed externally and in a spaced apart relationship with an inner side hull formed from existing side plating. The rebuilt double bottom hull is connected at each end (e.g., at the turn of the bilge) to the rebuilt double side hulls.
According to one aspect of the invention, a plurality of connecting members connect the new inner bottom hull and the existing outer bottom hull in a spaced apart relationship. In one preferred embodiment, the plurality of connecting members that connect the new inner bottom hull and the existing outer bottom hull include existing web framing. In this embodiment, the new inner bottom plating is laid on top of and connected directly to the existing web framing. In one embodiment, the existing web framing further comprises transverse stiffening members.
According to another aspect of the invention, the rebuilt double hull tanker includes a space formed between the new inner bottom hull and the existing outer bottom hull. Preferably, the bottom space comprises a distance H between the new inner bottom hull and the existing outer bottom hull measured at right angles, wherein H is not less than H=beam/15 or 2 meters, whichever is the lesser, and wherein the minimum value of H=1 meter.
According to another aspect of the invention, the rebuilt double hull tanker includes a plurality of connecting members connecting the existing inner side hull and the new outer side hull in a spaced apart relationship. In one embodiment, the side connecting members includes new connecting plates connected at a first end to the existing side plating of the existing inner side hull and connected at a second end to the new side plating of the new outer side hull. In another embodiment, the first end of the connecting plates are butt into the existing inner side hull plating in way of an existing supporting web frame and the second end of the connecting plates are lapped on a face of new vertical side shell stiffeners of the new outer side hull.
According to another aspect of the invention, a side space is formed between the existing inner side hull and the new outer side hull. Preferably, the minimum side spacing is based on the deadweight of the tanker and extends either for the full depth of the rebuilt double hull tanker""s side or from a top of the double bottom hull to a topside deck. The minimum side spacing is preferably defined by a distance W which is measured at any cross-section at right angles to the existing inner side hull and defined by W=0.5+deadweight/20,000(m) or 2 meters, whichever is the lesser, and wherein the minimum value of W=1 meter.
According to another aspect of the invention, temporary access holes are provided in the existing side hull at a location just above the existing stiffening members for installation of the new inner bottom hull over the existing outer bottom hull. Also, temporary access apertures can be provided in one or more longitudinal bulkheads to facilitate installation of the new inner bottom hull over a portion of the cargo hull from the side shell with the access holes to a longitudinal bulkhead in the way of one cargo hull. Inserts are used to renew the access holes and access apertures after installation of the new inner hull. Preferably, the cutout sections of the side shell and the lower portion of the longitudinal bulkheads are reused as the inserts.
Preferably, the temporary access holes are only cut on either a port side or a starboard side of the tanker at a time and in way of only one adjacent cargo hold at a time and the integrity of the opposite side of the tanker is kept intact to maintain the structural strength of the tanker. In embodiments where the tanker to be rebuilt includes multiple cargo holds, the new inner bottom hull can be installed simultaneously in more than one cargo hold with adjacent cargo holds being worked from alternative port and starboard sides of the tanker in order to retain structural integrity and sufficient strength during the installation process of the new inner bottom hull.
Preferably, the rebuilt double hull tanker maintains substantially the same cargo carrying capability as the existing single hull tanker. In one embodiment this can be accomplished by converting one or more existing ballast tanks to cargo tanks and using a space between one or both of the new inner bottom hull and the new outer bottom hull of the new double bottom hull and the new outer side hulls and the existing inner side hulls of the new side double hulls as new ballast tanks. In another embodiment, the rebuilt double hull tanker has an increased cargo carrying capability as compared to the existing single hull tanker.
The rebuilt double hull tanker preferably reuses the existing hull structure to the maximum extent possible. In one preferred embodiment substantially all of the existing hull structure is reused.
The draft of the rebuilt double hull tanker is preferably reduced for the same cargo load by installing the new double side hulls externally over the existing inner side hull. Installing the new outer side hull externally over the existing inner side hull results in an increase in the beam of the rebuilt double hull tanker and also an increase in the buoyancy for the rebuilt double hull tanker as compared to the existing single hull tanker.
In one preferred embodiment, a portion of the existing single hull is cut-away at a turn of the bilge. This facilitates the installation of the new inner hull through the side shell of the tanker. In one embodiment, new bottom filler pieces are connected to each outboard end of the new double bottom hull where the existing turn of the bilge was cut-away. Preferably, the new bottom filler pieces are scribed to match the existing outer bottom hull, including any dead rise, and directly support the inner side hulls. The cut-away portion of the turn of the bilge is preferably reused after installation of the new inner hull. The cut-away portion of the turn of the bilge is connected to an outboard end of the new bottom filler pieces. New outer side filler pieces including the new outer side hull are preferably connected over the exterior of the existing port and starboard inner side hulls and connected to the existing turn of the bilges. The new outer side filler pieces include new outer portions of topside deck plating that are preferably scribed out to match a contour of the shear strake of existing topside deck plating and that are connected to an outer periphery of the existing topside deck plating.
In accordance with another embodiment within the scope of the present invention, the rebuilt double hull tanker includes faired sections formed between the new double side hulls and the existing side hull of the single hull. The faired sections provide a relatively smooth transition between the new outer side hulls and the outer hull of the existing single hull proximate a bow section and a stern section for a smoothing hydrodynamic transition fore and aft in the area where the new double side hull and the existing single side hull meet. Preferably, the faired sections partially comprise an elastomer fairing compound.
The present invention also includes a method of rebuilding an existing single hull tanker into a rebuilt double hull tanker. The method includes forming a new double hull having a new double bottom hull and new side hulls connected at each outboard end of the new double bottom hull. The new double bottom hull is formed by disposing a new inner bottom hull through the side shell of the tanker internally over the existing outer bottom hull. The new double side hulls are formed by disposing a new outer side hull externally over the existing inner side hull. Preferably, the new double hull is formed over at least the cargo carrying portion of the tanker by installing the new inner bottom hull internally over the existing outer bottom hull through access holes cut into the sides of the tanker and installing the new double side hulls externally over the existing inner side hulls.
In accordance with another aspect of the invention, the method further comprises installing the new inner bottom hull simultaneously in more than one cargo hold with adjacent cargo holds being worked from alternative port and starboard sides of the tanker in order to retain structural integrity and sufficient strength during the installation process of the new inner bottom hull.
Additional features of the present invention are set forth below.